1. Field of the Invention
The present invention relates to drive control for an objective lens in an automatic focusing apparatus.
2. Related Background Art
U.S. Pat. Nos. 4,238,143 and 4,451,729 describe apparatuses wherein a moving speed of a focusing lens in an objective lens is controlled for focusing and the moving speed is reduced when the lens is moved to a position near the just focus position of an object.
Conventional automatic focusing cameras can employ photographing lenses having focal lengths of several tens of millimeters to several hundreds of millimeters. A photographing lens having a long focal length requires a large torque of a drive unit for driving the lens. However, a photographing lens having a short focal length requires a small torque of the drive unit. For this reason, it is undesirable to use a single drive control method for all photographing lenses. For example, when a lens requiring a large drive torque is attached to a camera having a drive control system for controlling energization of a motor so as to prevent the lens having a small drive torque from overrunning from a servo target (just focus position), the drive speed is decreased too early, so as to prolong time required for driving the lens to the servo target. As a result, the response characteristics of the drive unit are degraded.
In another known conventional automatic focusing apparatus, a focusing lens is driven by a motor on the basis of a defocus amount. The next focusing detection cycle is performed during or after driving of the focusing lens by the motor to obtain another defocus amount, and this defocus amount is then used for the next focusing detection cycle. The focusing lens is stopped when the defocus amount is regarded to fall within the predetermined range of in-focus values. Focusing detection cycles are continuously repeated in the above conventional apparatus. For example, one of the camera photographing modes is a so-called one-shot auto focus mode wherein a shutter release request is allowed only after an in-focus signal is generated by the automatic focusing apparatus. In this automatic focusing apparatus, the focusing detection operation prior to generation of the in-focus signal is performed to check only the focusing state. In an automatic focusing apparatus using a charge storage type element (e.g., a CCD) as a light-receiving element, charge storage time of the light-receiving element upon stopping of the photographing lens and processing time of an auto focus algorithm constitute delay time until the shutter release request is accepted. As a result, the response characteristics of the automatic focusing apparatus are degraded.